1. Field of the Invention
This invention relates to an optical system for a view finder and a lens element suitable for the view finder.
2. Description of Related Art
Inverted Galilean type of finder is well known as a view finder for cameras. It has a simple optical structure comprising two lens elements, namely an object lens having a negative refracting power and an eyepiece lens having a positive refracting power, and has a feature to be able to provide a large finder magnification by magnifying function of eyepiece lens and a wide field of view in spite of its simple optical structure. Therefore an inverted Galilean type of view finder is one of the most popular finders used for cameras, in particular, such as a low-cost 35 mm size lens shutter camera and a lens-fitted film unit, well known as a single-use or disposable camera, one of typical products of this kind having a taking lens with a focal length of about 32 mm and an angle of view of about 68 degrees.
Inverted Galilean type finder used in such a camera is usually 25.5 to 27.5 mm long in overall axial length of the optical system which refers a distance between an object side surface of the object lens and an image side surface of the eyepiece lens.
As plastic molding technology is developed, it has became possible to produce accurate light weight lenses with low costs on a large size. As known from, for example, Japanese Unexamined Patents Publications Nos. 50-87027 and 55-93116, and Japanese Patent Publications Nos. 61-40087 and 3-20732, inverted Galilean type finders have a distortion corrected up to about -4% by using an object lens with an aspherical surface formed on one of its sides.
Recently, an APS (Advanced Photo System) type cartridge has been introduced on the market. The cartridge has a function that a filmstrip entirely accommodated inside the cartridge before being loaded in the camera can be thrust out of the cartridge at need by rotating a spool of the cartridge in an unwinding direction. The APS filmstrip has a width of 24 mm and consequently a size of available image area for one shot is smaller than that of the conventional 35 mm size film. Moreover the size of an APS cartridge is lower in height and smaller in depth, which is always desirable to provide a camera with a depth much smaller as compared with using 35 mm type film because a shorter focal length taking lens can be employed.
In order to reduce the depth of camera, it is needed to incorporate not only a taking lens having a shorter focal length but a view finder having a shorter length. As is known from, for example, Japanese Unexamined Patent Publication No. 3-230276, an inverted Galilean type finder has a length between 20.43 and 20.63 mm. This fairly short length finder is realized by adopting an eyepiece lens of which one side is formed aspherical. However, the prior art finder is accompanied by aggravation of distortion increased up to -9%.
In the case where a concave lens having a strong curvature is formed on a rectangular plastic block, another problem arises as described below. In the inverted Galilean type finder, a finder magnification .beta. is given by the following expression (I), where f1 and f2 are focal lengths of an object lens and an eyepiece lens, respectively. EQU .beta.=-f1/f2 (I)
The overall length of the finder optical system D is given by the following expression (II), EQU D=f1+f2 (II)
Derived from the above expressions (I) and (II), the overall length of the finder optical system D is expressed as follows: EQU D=f1.times.(1-1/.beta.) (III)
It is found from the equation (III) that the smaller the focal length of the object lens f1 is, the shorter the overall length of the finder optical system becomes as long as the finder magnification .beta. remains constant.
A prior art rectangular block lens 5 as an object lens of a Galilean type view finder which is injection-molded, such as shown in FIGS. 25 and 26 by way of example, has a concave spherical lens surface 7 on the image side 6a of a rectangular-shaped solid lens base 6 having a plane face similar to an image area of the filmstrip. In order to make the focal length of the object lens 5 shorter, the concave spherical lens surface 7 must have a curvature sufficiently strong to provide a strong refracting power.
However, it brings, as shown in FIG. 26, the lens surface 7 is unavoidably increased in thickness Dp at the periphery in order to have a stronger curvature with the same effective radius Re while it has a strong curvature. As a result, the ratio of the peripheral thickness Dp to the central thickness Dc increases. In the plastic object lens 5 with the rectangular-shaped solid lens base 6, the volume of both side portions becomes much larger than that of the central portion of the lens. This will cause a substantial difference of a shrinkage of the injection-molded plastic resin between the side portions and the central portion and make the rectangular-shaped solid lens base 6 easy to produce a bend in the longitudinal direction, which makes it difficult to keep the lens surface 7 remain accurate.
There seems to be two ways to solve the problem that, while the ratio of the peripheral thickness Dp to the central thickness Dc is made closer to 1 (one), the aspect ratio of the rectangular-shaped solid lens base 6 is made closer to 1 (one). However to increase the central thickness Dc leads to a longer molding time which is always undesirable. In order for the rectangular-shaped solid lens base 6 to have an aspect ratio as close to 1 (one) as possible, it is necessary either to increase its height or to reduce its width. The former needs extra thick portions to upper and lower parts of the rectangular-shaped solid lens base 6 and results in having a larger size finder. The latter causes light reflection from the periphery of the lens to enter into a view which makes it hard for a user to see a subject clearly through the finder.